Test Systems are designed to diagnose production faults of assembled printed circuit (PC) boards which go into electronic products. Test equipment for testing various PC boards has been in use which allows in-circuit testing of the PC board, where testing is done on one element on the board to determine if a particular element on the board is working properly. In a different test of the PC board, functional testing of the PC board can be accomplished where the PC board is allowed to function as a whole to determine if the entire board is functioning properly. A typical test of a PC board then, includes incircuit testing which is then followed by a functional test of the PC board. This two-stage testing has heretofore typically been done on test equipment requiring two test fixtures or on an extensive; elaborate test fixture due to the following constraints.
In in-circuit testing, wires in the test equipment are used to connect signal sources and supply a stimulus to the device under test and a data response is carried back to the test system so that in in-circuit testing there is control over the signal source and the effects of a probe from the test system to the device under test are not of concern. In in-circuit testing there is maximum current flow through the wires, overdriving the device under test so that results at a previous node can be overridden.
In functional testing there is a stimulus supplied to the input end of the whole board under test and data is read at the output end of the board under test. Digital circuits are switching circuits which go from zero to one at a high clock rate and placing a probe on each point in the device under test will cause capacitive loading of the high clock rate digital circuit. Increased capacitance will load the circuit and will produce undesired responses on the rising and falling edges of digital signals so that the probe delays effects expected by the device under test and the fast edge response of the digital signal is altered. As the transient response observed at the device under test does not reach valid logic levels immediately, the delay can result in improper functional operation of the device under test. Also, wired in the test equipment are used to connect probes or sensors to the test system at different test points. Test signals are distorted by the impedance loading of the wires of the test equipment. Functional testing, therefore, should be done with as little loading effect as possible being perpetrated by the test instrument. To overcome this problem in functional testing, an inductor has been used in series with the device under test to change the resonant frequency so that signal quality is not affected by increased capacitance. Increased impedance to the test system and the wires minimizes the effects of the test system on the device under test and serves to isolate the device under test from the test system thereby allowing proper functional operation of the device under test.
Therefore what has been desired in the prior art is to reduce the capacitive loading of the test equipment on the device under test for functional testing and to still allow the test system to receive signals back from the device under test through the inductor for in-circuit testing.
However, the prior art implementation does not allow pulsing large direct current as generated by in-circuit testing. In the prior art, the inductor used in series with the device under test was so large that it did not saturate which delayed and distorted pulsing signals received in incircuit testing and therefore the inductor functioned to decouple the device under test from the test equipment in in-circuit testing.
Until now, these two tests, in-circuit testing and functional testing, have been performed by a test system having two test fixtures so that the loading effects of the in-circuit test system did not effect the functional testing of the PC board. This has been costly and inefficient, typically requiring the use of two test fixtures to properly test a PC board.
One manufacturer has attempted to minimize the problem by providing a two-stage test fixture whereby in-circuit testing is done on one plane with very short probes so that during in-circuit testing all the probes are connected to the board under test. For functional testing, the board under test is released half way up so that only longer probes remain in contact with the board under test to break the connection between the test system and the board under test so that there is no capacative loading. This still requires physical movement of the board under test to accomplish the two desired tests and requires the use of very expensive test equipment.